[boost] Re: Traits in general (Was: container traits (was: Pre-submission: container algorithms))

From: Valentin Bonnard (Bonnard.V_at_[hidden])
Date: 1999-07-08 20:39:33

Beman Dawes wrote:
> 
> At 05:05 PM 7/7/99 +0200, Valentin Bonnard wrote:
> 
> >// very useful example of the use of traits
> 
> Valentin, you lost me.

Sorry.

> Did you mean for the definition of foo to be
> preceded by template<class T>?  And isn't bar(Red()); etc, supposed
> to be foo(Red()); etc?
> I guess #include "fobar.hpp" is supposed to be
> #include "foobar.hpp"

yes, yes, and yes

> Plus you better include the source for
> sometrait.hpp.  Sorry to be old-fashioned, but I would actually like
> to compile and run your example to make sure I understand it.

OK (I really wrote the code fastly and it wasn't meant to compile)

With egcs, the following compiles and run:

// very useful example of the use of traits

// "sometrait.hpp"
struct yes {};
struct no {};
struct maybe {};

template <typename T>
struct sometrait {
    typedef maybe is_red;
};

// foobar.hpp
// #include "sometrait.hpp"

#include <iostream>

using std::cout;

void bar (maybe)
{
    cout << "color maybe\n";
}

void bar (yes)
{
    cout << "red\n";
}

void bar (no)
{
    cout << "non red\n";
}

template <typename T>
void foo (T)
{
    bar (sometrait<T>::is_red ());
}

// #include "fobar.hpp"

struct Red {};
struct Alien {};

template <>
struct sometrait<Red> {
    typedef yes is_red;
};

int main ()
{
    foo (Red ());
    foo (Alien ());
}

prints 

red
color maybe

-- 
Valentin Bonnard
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